Improve error messages for recursive GATs implicitly deriving Sized

compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs

@@ -55,6 +55,12 @@ pub trait InferCtxtExt<'tcx> {
 
     fn report_overflow_error_cycle(&self, cycle: &[PredicateObligation<'tcx>]) -> !;
 
+    fn maybe_report_recursive_gat(
+        &self,
+        begin: &PredicateObligation<'tcx>,
+        end: &PredicateObligation<'tcx>,
+    ) -> bool;
+
     /// The `root_obligation` parameter should be the `root_obligation` field
     /// from a `FulfillmentError`. If no `FulfillmentError` is available,
     /// then it should be the same as `obligation`.
@@ -213,6 +219,103 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
         bug!();
     }
 
+    fn maybe_report_recursive_gat(
+        &self,
+        begin: &PredicateObligation<'tcx>,
+        end: &PredicateObligation<'tcx>,
+    ) -> bool {
+        let (head, tail) =
+            match (begin.predicate.kind().skip_binder(), end.predicate.kind().skip_binder()) {
+                (ty::PredicateKind::Trait(head, ..), ty::PredicateKind::Trait(tail, ..)) => {
+                    (head, tail)
+                }
+                _ => return false,
+            };
+
+        if Some(head.clone().def_id()) != self.tcx.lang_items().sized_trait() {
+            return false;
+        }
+
+        let head_ty = head.self_ty();
+        let tail_ty = tail.self_ty();
+        debug!("report_overflow_error_cycle: head_ty = {:?}, tail_ty = {:?}", head_ty, tail_ty);
+
+        let head_kind = head_ty.kind();
+        let tail_kind = tail_ty.kind();
+        debug!(
+            "report_overflow_error_cycle: head_kind = {:?}, tail_kind = {:?}",
+            head_kind, tail_kind
+        );
+
+        let (substs, item_def_id) = match head_kind {
+            ty::Projection(ty::ProjectionTy { substs, item_def_id }) => (substs, item_def_id),
+            _ => return false,
+        };
+
+        let hir_id = self.tcx.hir().local_def_id_to_hir_id(item_def_id.expect_local());
+        let node = self.tcx.hir().get(hir_id);
+        debug!("report_overflow_error_cycle: node = {:?}", node);
+
+        let (assoc_generics, _assoc_span) = match node {
+            Node::TraitItem(hir::TraitItem {
+                generics,
+                kind: hir::TraitItemKind::Type(..),
+                span,
+                ..
+            }) => (generics, span),
+            _ => return false,
+        };
+
+        let parameter_size = assoc_generics.params.len();
+        if parameter_size == 0 {
+            return false;
+        }
+
+        assert!(substs.len() >= parameter_size);
+
+        let index = match substs
+            .get(substs.len() - parameter_size..)
+            .map(|gens| {
+                gens.iter()
+                    .enumerate()
+                    .find_map(|(i, param)| if param == &tail_ty.into() { Some(i) } else { None })
+            })
+            .flatten()
+        {
+            Some(index) => index,
+            None => return false,
+        };
+        debug!("report_overflow_error_cycle: index = {:?}", index);
+
+        let param = &assoc_generics.params[index];
+        debug!("report_overflow_error_cycle: param = {:?}", param);
+
+        let mut err = struct_span_err!(
+            self.tcx.sess,
+            begin.cause.span,
+            E0275,
+            "overflow evaluating the requirement `{}`",
+            head
+        );
+
+        err.note("detected recursive derive for `Sized` in a generic associated type");
+
+        let (span, separator) = match param.bounds {
+            [] => (param.span.shrink_to_hi(), ":"),
+            [.., bound] => (bound.span().shrink_to_hi(), " +"),
+        };
+        err.span_suggestion_verbose(
+            span,
+            "consider relaxing the implicit `Sized` restriction",
+            format!("{} ?Sized", separator),
+            Applicability::MachineApplicable,
+        );
+
+        err.emit();
+        self.tcx.sess.abort_if_errors();
+        true
+    }
+
     /// Reports that a cycle was detected which led to overflow and halts
     /// compilation. This is equivalent to `report_overflow_error` except
     /// that we can give a more helpful error message (and, in particular,
@@ -224,7 +327,13 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
 
         debug!("report_overflow_error_cycle: cycle={:?}", cycle);
 
-        self.report_overflow_error(&cycle[0], false);
+        let begin = &cycle[0];
+        let end = &cycle[cycle.len() - 1];
+
+        if !self.maybe_report_recursive_gat(begin, end) {
+            self.report_overflow_error(begin, false);
+        }
+        bug!()
     }
 
     fn report_selection_error(

compiler/rustc_trait_selection/src/traits/project.rs

@@ -327,8 +327,7 @@ impl<'a, 'b, 'tcx> AssocTypeNormalizer<'a, 'b, 'tcx> {
 
     fn fold<T: TypeFoldable<'tcx>>(&mut self, value: T) -> T {
         let value = self.selcx.infcx().resolve_vars_if_possible(value);
-        debug!(?value);
-
+        debug!(?value, "fold");
         assert!(
             !value.has_escaping_bound_vars(),
             "Normalizing {:?} without wrapping in a `Binder`",

compiler/rustc_trait_selection/src/traits/select/confirmation.rs

@@ -130,8 +130,12 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             let tcx = self.tcx();
 
             let trait_predicate = self.infcx.shallow_resolve(obligation.predicate);
+            debug!(?trait_predicate);
+
             let placeholder_trait_predicate =
                 self.infcx().replace_bound_vars_with_placeholders(trait_predicate);
+            debug!(?placeholder_trait_predicate);
+
             let placeholder_self_ty = placeholder_trait_predicate.self_ty();
             let (def_id, substs) = match *placeholder_self_ty.kind() {
                 ty::Projection(proj) => (proj.item_def_id, proj.substs),
@@ -140,9 +144,13 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             };
 
             let candidate_predicate = tcx.item_bounds(def_id)[idx].subst(tcx, substs);
+            debug!("candidate predicate before poly trait = {:?}", candidate_predicate);
+
             let candidate = candidate_predicate
                 .to_opt_poly_trait_ref()
                 .expect("projection candidate is not a trait predicate");
+            debug!("candidate predicate after poly trait = {:?}", candidate_predicate);
+
             let mut obligations = Vec::new();
             let candidate = normalize_with_depth_to(
                 self,
@@ -162,7 +170,11 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             })?);
 
             if let ty::Projection(..) = placeholder_self_ty.kind() {
-                for predicate in tcx.predicates_of(def_id).instantiate_own(tcx, substs).predicates {
+                let predicates = tcx.predicates_of(def_id);
+                debug!(?predicates);
+                let instantiated_predicates = predicates.instantiate_own(tcx, substs);
+                debug!(?instantiated_predicates);
+                for predicate in instantiated_predicates.predicates {
                     let normalized = normalize_with_depth_to(
                         self,
                         obligation.param_env,
@@ -179,6 +191,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                     ));
                 }
             }
+            debug!(?obligations);
 
             Ok(obligations)
         })

compiler/rustc_trait_selection/src/traits/select/mod.rs

@@ -868,7 +868,10 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
     where
         I: Iterator<Item = ty::Predicate<'tcx>>,
     {
-        cycle.all(|predicate| self.coinductive_predicate(predicate))
+        cycle.all(|predicate| {
+            debug!(?predicate, "coinductive_match");
+            self.coinductive_predicate(predicate)
+        })
     }
 
     fn coinductive_predicate(&self, predicate: ty::Predicate<'tcx>) -> bool {
@@ -1572,8 +1575,9 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
 
         // NOTE: binder moved to (*)
         let self_ty = self.infcx.shallow_resolve(obligation.predicate.skip_binder().self_ty());
+        debug!(?self_ty, "sized_conditions");
 
-        match self_ty.kind() {
+        match *self_ty.kind() {
             ty::Infer(ty::IntVar(_) | ty::FloatVar(_))
             | ty::Uint(_)
             | ty::Int(_)

compiler/rustc_ty_utils/src/ty.rs

@@ -193,7 +193,7 @@ fn adt_sized_constraint(tcx: TyCtxt<'_>, def_id: DefId) -> ty::AdtSizedConstrain
     let result = tcx.mk_type_list(
         def.variants
             .iter()
-            .flat_map(|v| v.fields.last())
+            .flat_map(|v| &v.fields)
             .flat_map(|f| sized_constraint_for_ty(tcx, def, tcx.type_of(f.did))),
     );
 

compiler/rustc_typeck/src/collect.rs

@@ -1978,6 +1978,7 @@ fn gather_explicit_predicates_of(tcx: TyCtxt<'_>, def_id: DefId) -> ty::GenericP
 
     let hir_id = tcx.hir().local_def_id_to_hir_id(def_id.expect_local());
     let node = tcx.hir().get(hir_id);
+    debug!("explicit_predicates_of: node = {:?}", node);
 
     let mut is_trait = None;
     let mut is_default_impl_trait = None;
@@ -2070,6 +2071,7 @@ fn gather_explicit_predicates_of(tcx: TyCtxt<'_>, def_id: DefId) -> ty::GenericP
     if let Some(_trait_ref) = is_trait {
         predicates.extend(tcx.super_predicates_of(def_id).predicates.iter().cloned());
     }
+    debug!("explicit_predicates_of: super predicates = {:?}", predicates);
 
     // In default impls, we can assume that the self type implements
     // the trait. So in:
@@ -2085,6 +2087,7 @@ fn gather_explicit_predicates_of(tcx: TyCtxt<'_>, def_id: DefId) -> ty::GenericP
             tcx.def_span(def_id),
         ));
     }
+    debug!("explicit_predicates_of: default impls = {:?}", predicates);
 
     // Collect the region predicates that were declared inline as
     // well. In the case of parameters declared on a fn or method, we
@@ -2113,6 +2116,8 @@ fn gather_explicit_predicates_of(tcx: TyCtxt<'_>, def_id: DefId) -> ty::GenericP
         }
     }
 
+    debug!("explicit_predicates_of: region predicates = {:?}", predicates);
+
     // Collect the predicates that were written inline by the user on each
     // type parameter (e.g., `<T: Foo>`).
     for param in ast_generics.params {
@@ -2132,6 +2137,7 @@ fn gather_explicit_predicates_of(tcx: TyCtxt<'_>, def_id: DefId) -> ty::GenericP
                     sized,
                     param.span,
                 );
+                debug!("explicit_predicates_of: bounds = {:?}", bounds);
                 predicates.extend(bounds.predicates(tcx, param_ty));
             }
             GenericParamKind::Const { .. } => {
@@ -2142,6 +2148,8 @@ fn gather_explicit_predicates_of(tcx: TyCtxt<'_>, def_id: DefId) -> ty::GenericP
         }
     }
 
+    debug!("explicit_predicates_of: inline predicates = {:?}", predicates);
+
     // Add in the bounds that appear in the where-clause.
     let where_clause = &ast_generics.where_clause;
     for predicate in where_clause.predicates {
@@ -2251,8 +2259,11 @@ fn gather_explicit_predicates_of(tcx: TyCtxt<'_>, def_id: DefId) -> ty::GenericP
         }
     }
 
+    debug!("explicit_predicates_of: where predicates = {:?}", predicates);
+
     if tcx.features().const_evaluatable_checked {
         predicates.extend(const_evaluatable_predicates_of(tcx, def_id.expect_local()));
+        debug!("explicit_predicates_of: const predicates = {:?}", predicates);
     }
 
     let mut predicates: Vec<_> = predicates.into_iter().collect();

src/test/ui/generic-associated-types/issue-80626-pass.rs

@@ -0,0 +1,60 @@
+#![feature(generic_associated_types)]
+
+// check-pass
+
+trait Allocator {
+    type Allocated<T: ?Sized>;
+}
+
+enum LinkedList<A: Allocator> {
+    Head,
+    Next(u8, A::Allocated<Self>),
+}
+
+enum LinkedList2<A: Allocator> {
+    Head,
+    Next(A::Allocated<Self>, u8),
+}
+
+impl Allocator for () {
+    type Allocated<T: ?Sized> = Box<T>;
+}
+
+fn main() {
+    {
+        use LinkedList::{Head, Next};
+        let mut ll: LinkedList<()> = Next(
+            8,
+            Box::new(Next(
+                0,
+                Box::new(Next(
+                    6,
+                    Box::new(Next(2, Box::new(Next(6, Box::new(Head))))),
+                )),
+            )),
+        );
+
+        while let Next(num, next) = ll {
+            println!("{}", num);
+            ll = *next;
+        }
+    }
+    {
+        use LinkedList2::{Head, Next};
+        let mut ll: LinkedList2<()> = Next(
+            Box::new(Next(
+                Box::new(Next(
+                    Box::new(Next(Box::new(Next(Box::new(Head), 6)), 2)),
+                    6,
+                )),
+                0,
+            )),
+            8,
+        );
+
+        while let Next(next, num) = ll {
+            println!("{}", num);
+            ll = *next;
+        }
+    }
+}